1. Field of the Invention
This invention relates to an automatic transmission system for a vehicle.
2. Description of the Prior Art
There has been known a belt-type automatic transmission system for a vehicle having a torque convertor and an infinitely variable gear which comprises a primary pulley mounted on a primary shaft, a secondary pulley mounted on a secondary shaft parallel to the primary shaft and an endless belt passed around the primary and secondary pulleys and in which the transmission ratio is infinitely changed by changing the ratio of the effective diameter of the primary pulley to that of the secondary pulley. (See, for instance, Japanese Unexamined Patent Publication No. 1(1989)-112076)
In such a belt-type automatic transmission system, the output of the transmission must be transmitted to the wheels through a reduction gear having a large reduction ratio which is provided between the output shaft of the transmission and a differential. The automatic transmission system is generally housed in a casing comprising an infinitely variable gear housing portion in which the infinitely variable gear is housed and a torque convertor housing portion in which the torque convertor is housed. One end of the shaft of the reduction gear is supported for rotation on the infinitely variable gear housing portion by way of a bearing and the other end of the reduction gear shaft is supported for rotation on the torque convertor housing portion by way of a bearing. The infinitely variable gear housing portion and the torque convertor housing portion are positioned relative to each other based on a knock pin so that the bearings for supporting opposite ends of the reduction gear shaft are centered relative to each other.
However when the reduction gear shaft is supported in such a manner, the bearings for supporting opposite ends of the reduction gear shaft cannot be accurately centered relative to each other due to dimensional errors of the infinitely variable gear housing portion and the torque convertor housing portion and the like and the reduction gears cannot mesh with each other in a desirable manner, whereby gear noise is produced.
In Japanese Patent Publication No. 3(1991)-72858, there is disclosed a structure for supporting the primary and secondary shafts of the infinitely variable gear. In the structure, one end of the primary shaft which is connected to the torque convertor is supported on the torque convertor housing portion of the transmission casing and the other end of the primary shaft is supported on the infinitely variable gear housing portion. One end of the secondary shaft which is connected to the reduction gear is supported on the infinitely variable gear housing portion and the other end of the secondary shaft is supported on a partition wall member which is provided separately from the torque convertor housing portion and the infinitely variable gear housing portion. The partition wall member is disposed to separate the infinitely variable gear and the reduction gear from each other and is provided with a through hole through which the other end portion of the secondary shaft extends to be connected to the reduction gear. The other end of the secondary shaft is supported by a bearing provided in the through hole.
In the automatic transmission system disclosed in Japanese Unexamined Patent Publication No. 57(1982)-173652, a forward-reverse switching mechanism is carried by the secondary shaft of the infinitely variable gear.
In the conventional system described above, in order to rigidly incorporate the transmission casing, the partition wall member must be rigidly incorporated as well as the infinitely variable gear housing portion and the torque convertor housing portion. That is, the partition wall member is not only for separating a space but also for positioning and supporting in place the secondary shaft. Accordingly, the partition wall member must be rigidly incorporated with a substantial mounting accuracy, which adversely affects the incorporating facility of the transmission casing.
Further the partition wall member makes it difficult to incorporate the reduction gear to be disposed inside the partition wall member.
Further when the primary shaft and the secondary shaft are positioned to be parallel to each other, the positioning accuracy of the end of the secondary shaft on the side of the torque convertor housing portion is affected by the sum of the dimensional error of the torque convertor housing portion and the mounting error of the partition wall member and accordingly the secondary shaft cannot be positioned with a high accuracy relative to the primary shaft.
Further since the end of the secondary shaft is extended beyond the bearing provided in the partition wall member and a gear in mesh with a gear of the reduction gear is mounted on the extreme end of the secondary shaft, the end portion on the side of the reduction gear is supported by the bearing in a cantilever fashion. In such a state, it is difficult to stably support the reduction gear.
Further since the secondary shaft involves problems in incorporating facility and/or mounting accuracy as described above, it is difficult to position the forward-reverse switching mechanism with a high accuracy relative the parts to be associated therewith when the forward-reverse switching mechanism is mounted on the secondary shaft.